This invention is a processing circuit for integrating electrical input signals over an adjustable period of time for the purpose of noise suppression.
For noise suppression in very noisy signals it is a known practice to integrate the signal for a predetermined time period so as to suppress the noise by averaging it out over the integration period; assuming the signal amplitude remains substantially constant during the integration period, the integral is then directly proportional to the signal amplitude.
Noise suppression by integrating for a predetermined period is particularly useful in atomic absorption spectrometers that determine the concentration of a specific element in a sample mixture by spraying the sample into a flame which atomizes it. A light beam containing the spectral lines of the element of interest is directed through the flame. Light radiation in the spectrum of the element is absorbed thereby with the amount of absorption, which is detected by a photodetector and associated signal processing circuitry, being proportional to the concentration of that element in the sample. The signals produced by the photodetector and the signal processing circuitry are very noisy so that some sort of noise suppressing system is necessary in order to measure the absorption with a reasonable degree of accuracy.
Known integrators for noise suppression, such as Miller integrators, customarily consist of an operational amplifier with an ohmic resistor in the input circuit and a negative feedback through a capacitor. However, integrator circuits of this type are prone to variation of their integration constants, due particularly to variations in the capacitance of the negative feedback capacitor and these variations degrade the accuracy of measurement in the prior art where the integration is carried out over a fixed period.